3Com S7906E Configuration Manual page 1625

Figure 1-2 MAC learning and flooding on PEs
VSI
VPN 1
VPN 1
MAC A IP 1.1.1.2
PE 2
VSI MAC
VPN 1 A
VPN 1 B
2) MAC address reclaim
Dynamic address learning must support refreshing and relearning. The VPLS draft defines a dynamic
address learning method that uses the address reclaim message, which carries MAC TLV. Upon
receiving such a message, a device removes MAC addresses or relearns them according to the
specified parameters in the TLV. If NULL is specified, the device removes all MAC addresses of the VSI
except for those learned from the PW that received the address reclaim message.
The address reclaim message is very useful when the network topology changes and it is required to
remove the learned MAC addresses quickly. There are two types of address reclaim messages: those
with MAC address lists and those without MAC address lists.
After a backup link becomes active and a message with the instruction of relearning MAC entries
arrives, a PE updates the corresponding MAC entries in the FIB table of the VPLS instance and sends
the message to other PEs that are directly connected through LDP sessions. If the message contains a
null MAC address TLV list, these PEs remove all MAC addresses from the specified VSI, except for
those learned from the PW that sent the message.
3) MAC address aging
Remote MAC addresses learned by a PE that are related to VC labels but no more in use need to be
aged out by an aging mechanism. The aging mechanism used here is the aging timer corresponding to
the MAC address. When receiving a packet whose source MAC address has an aging timer started, the
PE resets the aging timer.
VPLS loop avoidance
In general, Layer 2 networks use the Spanning Tree Protocol (STP) to avoid loops. This is not
applicable for VPLS networks because the users cannot sense the service provider network. Therefore,
enabling STP in the private networks means nothing to the service provider network. In VPLS, full mesh
and split horizon forwarding are used to avoid use of STP at the private network side.
Two methods for VPLS loop avoidance are supported:
PE 1
MAC Port
A Vlan 10, port 1
B PW 1
PE 1
PW 3
PW 2
PW 1
Port
PW 1
Vlan 10, port 1
MAC B IP 1.1.1.3
ARP broadcast
ARP response
PE 3
PE 3
VSI MAC
PE 2
VPN 1 A
1-4
Port
PW 3